1. Field of the Invention
The invention relates to new epoxy resins and their use.
2. Description of Related Art
Non-linear optics deals with the interaction of the electromagnetic field of a light wave spreading in a medium with this medium, as well as with the related occurrence of new fields with changed properties. Specifically, if the electromagnetic field enters into interaction with the medium, which consists of one molecule or of many molecules, then this field polarizes the molecules.
Polarization, which is induced by a local electrical field in a molecule, can be represented as the power series of the electrical field intensity--corresponding to Equation (1): EQU P=.alpha..E+.beta..E.sup.2 +.gamma..E.sup.3 + . . . (1);
P is the induced polarization and E is the induced local electrical field, and .alpha., .beta. and .gamma. represent the polarizability of the first, second and third order.
On a macroscopic level, a similar relation holds true--according to Equation (2)--for polarization induced, by an external electrical field, in a medium consisting of several molecules: EQU P=.epsilon..sub.o (.chi..sup.(1).E+.chi..sup.(2).E.sup.2 +.chi..sup.(3).E.sup.3 +. . . ) (2);
again, P is the induced polarization and E is the induced local electrical field, .epsilon..sub.o is the dielectric constant, and .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) represent the dielectric susceptibility of the first, second and third order.
The dielectric susceptibilities in Equation (2) have a meaning similar to that of the molecular coefficients in Equation (1): They are material constants, which are dependent on the molecular structure and the frequency, and, in general also on the temperature. The coefficients .chi..sup.(2) and .chi..sup.(3) cause a great number of non-linear optical effects, specifically depending on the input frequency and the distance of the molecular oscillation frequencies or electronic resonances, and the input frequencies or frequency combinations, as well as the phase adaptation conditions.
Materials with a dielectric susceptibility of the second order are suited for frequency doubling (SHG=Second Harmonic Generation); this is the transformation of light with a frequency .omega. into light with a frequency 2.omega.. Another non-linear optical effect of the second order is the linear electro-optical effect (Pockels Effect); it results from the change in the index of refraction of the optical medium when an electrical field is applied. Optical rectification as well as sum and difference frequency mixing are further examples of non-linear optical effects of the second order.
Areas of use for materials of the type stated above are, for example, electro-optical switches as well as areas of data processing and integrated optics, such as optical chip-to-chip connections, wave-guiding in electro-optical layers, Mach Zehnder interferometers and optical signal processing in sensor technology.
Materials with a dielectric susceptibility of the third order are suited for frequency tripling of the incident light wave. Additional effects of the third order are optical bistability and phase conjugation. Concrete application examples are purely optical switches for constructing purely optical computers and holographic data processing.
To achieve a sufficient non-linear optical effect of the second order, the dielectric susceptibility of the second order .chi..sup.(2) must be greater than 10.sup.-9 electrostatic units (esu); this means that the hyperpolarizability .beta. must be greater than 10.sup.-30 esu. Another fundamental prerequisite for achieving a non-linear optical effect of the second order is the non-centrosymmetrical orientation of the molecules in the non-linear optical medium; otherwise, .chi..sup.(2) =0. This can be achieved with an orientation of the molecular dipoles, if it is not predetermined by the crystal structure, as in the case of crystalline materials. Thus, the greatest values for .chi..sup.(2) for a non-linear optical medium have been achieved by orientation of the molecular dipoles in electrical fields.
Inorganic materials, such as lithium niobate (LiNbO.sub.3) and potassium dihydrogen phosphate (KH.sub.2 PO.sub.4), demonstrate non-linear optical properties. Semiconductor materials, such as gallium arsenide (GaAs), gallium phosphide (GaP) and indium antimonide (InSb), also demonstrate non-linear optical properties.
However, along with the advantage of a high electro-optical coefficient of the second order, inorganic materials of the type stated have some major disadvantages. For example, the processing of these materials is very complicated in terms of technology, since individual process steps are time-consuming and must be carried out with extremely high accuracy (see in this regard: C. Flytzanis and J. L. Oudar "Nonlinear Optics: Materials and Devices," Springer-Verlag (1986), pages 2 to 30). These materials are furthermore unsuitable for those electro-optical components which work at high modulation frequencies. Due to the high dielectric constants which intrinsically present, the dielectric losses which occur at high frequencies (above several GHz) are so high that working at these frequencies is impossible (see in this regard: "J. Opt. Soc. Am. B," Vol. 6 (1989), pages 685 to 692).
It is known that organic and polymer materials with extended .pi. electron systems, which are substituted with electron donors and acceptors, demonstrate non-linear optical properties, i.e. can be used in non-linear optical media (see in this regard: R. A. Hann and D. Bloor "Organic Materials for Non-linear Optics," The Royal Society of Chemistry (1989), pages 382 to 389 and 404 to 411).
Monocrystals on an organic basis demonstrate a high electro-optical coefficient of the second order and good photochemical stability, in comparison with LiNbO.sub.3 ; the required high level of orientation of the non-linear optical molecules is also already present. Some significant criteria, however, speak against technical utilization of this material class. For example, production of the monocrystals, specifically both from solution and from a melt, requires a time of 14 to 30 days (see in this regard: D. S. Chemla and J. Zyss "Nonlinear Optical Properties of Organic Molecules and Crystals," Academic Press, Inc. (1987), Vol. 1, pages 297 to 356); the production process therefore does not meet the requirements of technical production. Furthermore, the melting point of the crystals lies at 100.degree. C., on the average, so that it would not be possible to achieve a working temperature range up to 90.degree. C. Furthermore, organic crystals cannot be structured and their lateral dimensions are presently still too small to allow them to be constructed as an electro-optical component.
For applications of non-linear optics in the areas of data transmission and integrated optics, polymer materials have found increasing importance recently. For this purpose, an external electrical field is applied to polymer materials heated above the glass transition temperature; this results in orientation of the non-linear optical molecules. After cooling (below the glass transition temperature), with the electrical field applied, anisotropic and therefore non-centrosymmetrical polymers are obtained, which demonstrate dielectric susceptibilities of the second order.
Non-linear optical compounds which are dissolved or diffused in polymers can be processed to form thin layers, in this manner, as is required by integrated optics (see in this regard: "Macromolecules," Vol. 15 (1982), pages 1385 to 1389; "Appl. Phys. Lett.," Vol. 49 (1986), pages 248 to 250; "Electron. Lett.," Vol. 23 (1987), pages 700 and 701). However, the low solubility of the compounds with a low molecular weight, their insufficient distribution in the polymers, the migration of the active molecules out of the polymer matrix and the loss of the non-centrosymmetrical orientation of the active molecule species over a period of only a few hours, even at room temperature, are disadvantageous in this connection.
Polymers with covalently bonded non-linear optical molecule components, which simultaneously have a liquid-crystalline character, are also known as non-linear optical compounds (see in this regard: EP-OS 0 231 770 and EP-OS 0 262 680). While these materials do not demonstrate the disadvantages stated above, they are not suited for applications in electro-optics and integrated optics in their current stage of development, since optical losses &gt;20 dB/cm, caused by the inherent domain scattering, occur here. Furthermore, studies of amorphous non-linear optical polymers have already been reported (see: "Macromolecules," Vol. 21 (1988), pages 2899 to 2901).
Both with liquid-crystalline polymers and with amorphous polymers with covalently bonded non-linear optical molecule units, a high concentration of such units can be achieved. A spacer thereby uncouples the molecular mobility of the non-linear linear optical units from the polymer chain; at the same time, however, the glass transition temperature decreases drastically. However, with this, a loss of the molecular orientation of the non-linear optical molecule units and a loss of the non-linear optical activity must be expected at use temperatures in the range of the glass transition temperature of the polymers.